1. Field of the Invention
The present invention relates to a transparent coextruded, multilayer polyolefin film which can be heat-sealed at low temperatures, comprising a base layer and at least one top layer applied on one side. The multilayer film is distinguished by high shrinkage in the longitudinal and transverse directions in combination with excellent optical properties and smooth passage through a packaging machine. The invention furthermore relates to a process for the production of a film of this type and to the use of the film.
2. Description of Related Art
Various types of shrink films are known from the prior art and are widely used as packaging films. Shrink packaging is usually produced starting from a pouch, into which the goods are introduced, followed by separation/welding and subsequent shrinkage in a shrink oven. In this way, very tight, fold-free film packaging is obtained, as is particularly desirable in many cases. In order that this tight wrap is reliably achieved, the films used must have high shrinkage values, since the film wrapping after separation/welding surrounds the contents relatively loosely. This is due to the fact that the welding wire cannot be moved right up against the packaged goods. High shrinkage values of the films are therefore a prerequisite for their use as conventional shrink packaging. Such films are described, for example, in EP-A-0 436 196, DE-A-19 54 467, DE-A-32 15 120, and EP-A-0 350 859.
Various types of heat-sealable films are likewise widespread and generally used as packaging materials. The heat-sealability of the material enables packaging of goods on high-speed packaging machines, which enables considerable cost advantages to be achieved.
Heat-sealable, biaxially oriented, multilayer polyolefin films in which the base layer comprises propylene homopolymers and the heat-sealable layer(s) (outer layer(s) or top layer(s)) comprises heat-sealable olefin polymers are described in numerous publications, for example, EP-A-0 194 588, EP-A-0 008 904, and U.S. Pat. No. 4,419,411. These multilayer polyolefin films have important properties for packaging films, namely a broad heat-sealing range, good heat-sealability, relatively high scratch resistance, low friction and thus good running properties on high-speed packaging machines of various types.
The heat-sealable films described are generally produced so that they are as dimensionally stable as possible, i.e., these films have the lowest possible shrinkage in the longitudinal and transverse directions. In general, packaging films of this type exhibit shrinkage values at 120.degree. C. of less than 5% in the longitudinal direction and less than 4% in the transverse direction. The prevailing opinion hitherto was that films having these low shrinkage values give a visually attractive wrap. In fact, however, the packages are wrapped in a loose or bloused manner and have folds and billows at the edges and are therefore unsuitable for goods in which particularly tight packaging is required.
In the search for heat-sealable shrink films it has been found that conventional shrink films are unsuitable for a heat-sealable shrink film. Although coating of known highly shrinkable films with a heat-sealable layer enables them to be used in packaging machines, the pronounced shrink properties, which ensure the desired tightness of the packaging, also cause the seal seams to shrink so much that the visual appearance of the packaging is changed in an unacceptable manner.
EP-A-489 373 discloses heat-sealable shrink films built up from a polypropylene (PP) homopolymer base layer and ethylene/propylene copolymer top layers. These films have increased shrinkage compared with conventional heat-sealable packaging films, but the shrink properties are inadequate if tight coverings are required.